In manufacture of semiconductor such as LSI and super LSI or in manufacture of a liquid crystal display or the like, a pattern is made by irradiating light to a semiconductor wafer or an original plate for liquid crystal, but if a dust adheres to a photomask or a reticle (hereinafter simply referred to as a photomask in short) used at this time, the dust absorbs light or refracts light, which causes deformation of a transferred pattern, rough edges or black stains on a base and leads to a problem that dimensions, quality, appearance and the like are damaged.
Thus, these works are usually performed in a clean room, but it is still difficult to keep the photomask clean all the time. Therefore, a pellicle is attached to the surface of the photomask as a dustproof and then, exposure is carried out. In this case, foreign substances do not directly adhere to the surface of the photomask but to the pellicle, and by setting a focus on a pattern of the photomask in lithography, the foreign substances on the pellicle no longer become a concern to the transfer.
In general, a transparent pellicle film made of cellulose nitrate, cellulose acetate, fluorine contained resin or the like, which transmits light well, is attached or bonded to an upper end face of a pellicle frame made of aluminum, stainless, polyethylene and the like. Moreover, a pressure-sensitive adhesive layer made of polybutene resin, polyvinyl acetate resin, acrylic resin and the like for attaching to a photomask and a separation layer (separator) for protecting the pressure-sensitive adhesive layer are disposed at a lower end of the pellicle frame.
Also, in a state in which the pellicle is attached to the photomask, with the purpose of eliminating an atmospheric pressure difference between a space surrounded inside the pellicle and the outside, a small hole for adjusting atmospheric pressure is opened in a part of the pellicle frame, and a filter is installed in order to prevent entrance of foreign substances from the air moving through the small hole in some cases (See Japanese Utility Model Registration Application Publication No. 63-39703).
With regard to this filter, a trapping diameter and an area are usually considered so that desired foreign-substance trapping performances and ventilation performances are obtained, and if the desired ventilation performances cannot be obtained with a single filter, a plurality of filters are installed. For example, the number of pellicles used for semiconductor manufacture is often one, but in the case of a large-sized pellicle for liquid crystal manufacture such as those having a side exceeding 500 mm, eight to several tens of filters are usually attached in order to ensure a ventilation amount.
As the filter used here, a sheet-shaped one in which a porous thin film obtained by stretching and working PTFE or the like is used as a ventilation film is suitable for use in the pellicle in general in terms of cost and the shape and often used. The structure is a three-layered structure as shown in FIG. 4, in which a pressure-sensitive adhesive layer 43 to be bonded to a frame is arranged in a ring shape on one face of a filter ventilation film 41, and a rough mesh-state protective mesh 42 made of PP or the like in order to protect the filter ventilation film 41 is bonded to the opposite face, and its entire thickness is approximately 0.15 to 0.3 mm. To this filter, an adhesive of permanent stickiness may be applied or infiltrated, so as to prevent dust generation in some cases (See Japanese Patent Laid-Open No. 09-068792).
In general, the pellicle frame is washed by means such as ultrasonic washing or the like in surfactant or purified water. At that time, the inside of the ventilation hole is also washed by ultrasonic washing in the liquid, but since an ultrasonic wave hardly enters the inside of the ventilation hole, a washing effect is lower than the usual pellicle frame surfaces (other than the hole). Particularly, in the case of a large-sized pellicle with one side exceeding 500 mm, since the frame width is large, the ventilation hole becomes longer with that, and the washing performance is extremely lowered. Moreover, in a foreign substance inspection of the inside of the ventilation hole after the washing, there is disadvantage that the inside of the ventilation hole can rarely be observed visually.
Moreover, the filter is usually mounted to the outer surface of the pellicle frame so as to cover the ventilation hole. Therefore, if a film surface is vibrated by air blow to the filter for example, there is a fear that the film surface is brought into contact with a peripheral edge portion of the ventilation hole and is damaged and generates dusts. The dust generation at this portion might cause a big problem of adhesion of a foreign substance to a mask pattern since it is within an inside area of the pellicle.